The combustion of a fuel, such as coal, oil, natural gas, peat, waste, and the like, in a combustion plant such as a power plant, generates a hot process gas stream known as a flue gas stream. In general, the flue gas stream contains particulates and gaseous contaminants such as carbon dioxide (CO2), nitrogen oxides (NOx) such as nitric oxide (NO) and nitrogen dioxide (NO2), nitrous oxide (N2O) and sulfur dioxide (SO2). The negative environmental effects of releasing these gaseous contaminants into the atmosphere have been recognized, and have resulted in the development of processes adapted for removing or reducing the amount of such gaseous contaminants from the flue gas streams.
Various combustion modification techniques have been developed to control the formation of NOx in flue gas streams. These techniques generally have relatively low NOx reduction efficiencies and involve significant heat loss. Flue gas stream treatment technologies can achieve significantly higher removal efficiencies than combustion modification techniques. Such flue gas treatment technologies include selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR). SCR and SNCR technologies utilize ammonia or urea to carry out chemical redox reactions to reduce NOx to nitrogen (N2) and water (H2O). However, significant drawbacks are associated with these technologies, such as high costs, catalyst degradation, ammonia slip and facility space in the case of SCR and a narrow temperature window and ammonia slip (unreacted ammonia) in the case of SNCR. Scrubbing systems which chemically absorb NOx from a flue gas stream offer an alternative to SCR and SNCR technologies and their associated drawbacks.
Recirculation is a technology which can be employed basically for the most diverse possible purposes in gas turbines. In recirculation of exhaust gases in a gas turbine, a substantial fraction of the exhaust gas is branched off from the overall exhaust gas substream and is normally delivered again, after cooling and purification, to the mass entry stream of the turbine or to the turbine compressor. The exhaust gas composition differs considerably from the composition of fresh ambient air. Conventionally, the recirculated exhaust gas substream is mixed with fresh air from the surroundings and this mixture is subsequently delivered to the compressor.
A gas treatment system is disclosed in WO2013103990A2, which includes a heat exchanger having a first side and a second side separated from one another. The first side defines a first inlet and a first outlet and the second side defines a second inlet and a second outlet. A direct contact cooler is in fluid communication with the first outlet, a direct contact heater is in fluid communication with the first inlet and/or a gas polisher is in fluid communication with the first inlet and the first outlet. The gas treatment system includes an ammonia polishing system in fluid communication with the second inlet and/or the second outlet. A possibility to suppress such oscillations consists in attaching damping devices, such as quarter wave tubes, Helmholtz dampers or acoustic screens.
An apparatus for reducing emissions and method of assembly is disclosed in US 20120102913 A1, wherein A heat recovery steam generator (HRSG) is coupled to a gas turbine engine that discharges a flow of exhaust gases including oxides of nitrogen (NOx). The HRSG includes a steam-based heating element for heating the exhaust gases, and at least one NOx reduction element coupled downstream from the at least one steam-based heating element and configured to facilitate reducing an amount of NOx in the exhaust gases that are channeled into the at least one NOx reduction element.
A method for applying ozone NOx control to an HRSG for a fossil fuel turbine application is disclosed in WO 2012094362 A3, wherein a method for reducing NOx and recovering waste heat from a stream of exhaust gas from a fossil fuel fired turbine includes contacting the stream of exhaust gas between an economizer and an evaporator with ozone gas to convert the NO to nitrogen dioxide (NO2) thereby forming a stream of exhaust gas comprising NO2 and residual NO. The method further includes, contacting the stream of exhaust gas comprising NO2 and residual NO with water mist to create an exhaust stream comprising nitric acid (HNO3) and residual NO. The method further includes cooling the stream of exhaust gas comprising HNO3 and residual NO, collecting a first residual water film on a first condensing medium to capture the HNO3 and removing the first water film and HNO3.
Even though great development has been achieve in this field, there still need further space to explore possible approaches to reduce NOx in the exhaust gas with lower cost and higher efficiency.